Supplemental vehicle heating method and apparatus with long heating cycle

ABSTRACT

An auxiliary burner provides supplemental thermal energy to a vehicle having a room and domestic water to be heated. The auxiliary burner operates efficiently at a rated thermal output when it is operated for at least the duration of a minimum operational cycle. A reservoir contains heat transfer liquid having a selected thermal energy storage capacity, measured by the amount of thermal energy required to heat the liquid from a minimum to a maximum operating temperature. The heat transfer liquid receives heat from the auxiliary burner. The thermal energy storage capacity is selected so that the auxiliary burner must operate at its rated thermal output for at least the duration of the minimum desired operational cycle to provide the selected thermal energy storage capacity to the reservoir. Methods of the present invention include heating the reservoir containing the heat transfer liquid having the selected thermal energy storage capacity by operating the auxiliary burner at the rated thermal output for at least the duration of the minimum desired operational cycle. With the heat transfer liquid in the thermal reservoir at the maximum operating temperature and the auxiliary burner off, heat is transferred from the thermal reservoir to meet thermal demands for heated room air and domestic hot water. These demands decrease the temperature of the heat transfer liquid to the minimum operating temperature, whereupon the auxiliary burner is operated for at least the duration of the minimum desires operational cycle to raise the temperature of the heat transfer liquid to the maximum operating temperature.

REFERENCE TO PARENT APPLICATION

This application is a division of application of H. R. Enander, Ser. No.07/318,392, filed Mar. 2, 1989, For SUPPLEMENTAL VEHICLE HEATING METHODAND APPARATUS WITH LONG HEATING CYCLE.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to providing supplemental thermal energy tovehicles having living areas, and more particularly to providingsupplemental heat to over-the-road and marine craft having rooms anddomestic water to be heated.

2. Discussion of Prior Art

Vehicles of many types have been used to provide temporary living orworking quarters. These include self-propelled over-the-road vehicles,such as so-called recreational vehicles powered by internal combustionengines. Also, self-propelled vans have been used as mobile work spaces,such as for providing medical services at remote or movable locations ina city. Other self-propelled vehicles include boats in which internalcombustion engines provide the primary power source. Othernon-self-propelled vehicles, such as trailers, have been used to provideshelter for temporary living, such as for vacation or recreation. Also,trailers are used to provide space for performing work, such as atconstruction sites or performing atmospheric sensing at remotelocations.

All of these vehicles are characterized by the need to provide heatedspace, in the form of at least one room. In general, many separate roomsor work areas are provided. Also, sanitary facilities are provided insuch vehicles, and include plumbing fixtures such as sinks, showers, andtoilets that use domestic water, especially heated water.

The term "vehicle" is used herein to refer to all of these types ofvehicles, whether or not self-propelled and whether an over-the-road orwater vehicle, so long as there is a space to be heated in the vehicleand/or a requirement that heated domestic water be available for use.The term "vehicle" may also include the above type of vehicle that isalso provided with a main power source, such as an internal combustionengine, that has a primary function of propelling the vehicle on land orwater. Customarily, those main power sources are heated when not inoperation, so that they will start readily when the vehicle is to bemoved.

The main power source of such vehicle is turned off when the vehiclearrives at the destination, and reliance is placed on a supplementalsource of thermal energy. Such supplemental thermal energy sourcesinclude diesel-fired and gasoline-fired burners, such as those disclosedin U.S. Pat. Nos. 2,726,042 and 3,877,639. These supplemental burnerstransfer heat to a small mass or volume of liquid in a thin jacket thatsurrounds the burner's combustion chamber. The thin jacket substantiallylimits the mass and volume of liquid that is in heat transferrelationship with the combustion chamber.

In applicant's experience with such burners, the liquid volume in thethin jackets is only about 0.26 gallons, and a typical system, includingconduits and liquid-to-air heat exchangers, may only have a two andone-half gallon liquid volume, including the 0.26 gallons in the thinjackets. Further, such mass and volume of the liquid in the thin jacketsare not selected according to the operating characteristics of theauxiliary burners.

Other systems used with such vehicles include relatively small domestichot water tanks, such as those holding eleven gallons. The domesticwater is heated by a heat exchanger that uses energy from the mainengine. Alternatively, the domestic water is heated when thesupplemental burner is operated for heating room air. The operation ofthe burner is not generally controlled in response to the demand for hotdomestic water. For example, U.S. Pat. No. 236,849 issued in 1881discloses pipes that supply heat to both a domestic water tank andradiators for room air heating.

SUMMARY OF THE INVENTION

In applicant's experience, there are problems with such supplementalburners. These problems are severe when attempts are made to divide thehot liquid from the thin jacket into separate heating zones, and tosupply heat to less than all of the zones at any given moment. Theburners inherently have a three-stage operational cycle, including aninitial, relatively inefficient, start-up stage in which thermal energyis used to heat the thermal mass of the burner, the combustion chamberand the exhaust system. The start-up is characterized by carbon build upon the inside walls of the combustion chamber due to incompletecombustion.

Once the combustion chamber has reached a normal, steady-state operatingtemperature (e.g., after three minutes of operation in the start-upstage--see t₀ to t₃ in FIG. 6), the burner operates more efficiently andstarts to burn the deposited carbon off the walls of the combustionchamber. This optimum stage efficiently converts the fuel to thermalenergy that is transferred to the liquid in the thin jacket (see t₃ tot₁₀ in FIG. 6).

When a condition to be controlled occurs, the burner is shut off (seeshut off at time t₁₀ in FIG. 6), and a purge stage starts. When there isthermal demand after the burner is shut off, the purge stage continuesfor less than the duration (2.5 minutes) of the normal purge stage andis thus abbreviated because a low limit temperature T_(L) is reachedbefore the purge stage ends. In the purge stage, air continues to flowthrough the combustion chamber and the liquid continues to flow in thethin jacket.

These prior art burners have a British thermal unit (BTU) outputsufficient to supply adequate heat to all of the zones at the same time.In applicant's experience, with a typical flow rate of 4.5 gallons perminute (gpm) through the thin jacket, the liquid temperature willincrease up to 18° F. in one pass through the thin jacket in the optimumstage of operation. In the typical prior art example illustrated in FIG.6, there are a total of two and one-half gallons of liquid in the thinjacket, conduits and heat exchangers, and the thin jacket weighs 15pounds. The normal factory temperature differential between T_(M) andT_(L) is 15° F. In this situation, when less than all of the zones areoperational or "active," the liquid flowing through the liquid-to-airheat exchangers in the active zones will generally not decrease theliquid temperature by the full 18° F. amount. As a result, as the liquidcirculates through the one active zone (or through less than all of thezones) and through the thin jacket, applicant has observed that theoutput temperature (see arrow T_(LO) in FIG. 6) of the liquid exitingthe thin jacket rises (see t₁₂.3 to t₁₃.5 in FIG. 6), and soon reachesan upper limit T_(M) at which the burner is shut off. Applicant'sexperience is that this shut off occurs very soon after t₁₂.3 (e.g., att₁₃.5, or 1.2 minutes after t₁₂.3 as shown in FIG. 6). In the situationdepicted in FIG. 6, where one heating zone always has a thermal demandof 14,000 BTU per hour, such shut off occurs before the burner reachesthe optimum stage, such that the carbon that built up in the combustionchamber during the start-up stage is not burned off because the optimumstage is not reached. Clearly, where the thermal demand is less than14,000 BTU per hour (e.g., for heating small amounts of domestic hotwater), the burner shut off occurs even sooner. If the thermal energystorage capacity of such burners is defined as the amount of thermalenergy required to heat a given number of gallons of the gallons of theliquid through a selected temperature change ΔT, then that thermalenergy storage capacity is small. Additionally, during a normal durationpurge stage, which may be 2.5 minutes, the burner chamber, the exhaustsystem and the thin jacket cool and are subject to thermal stresses fromthe repeated cooling of the structure, which decrease the operating lifeof the burner and the thin jacket.

Applicant has observed that as the two and one-half gallons of liquidcontinue to circulate through the single active zone (or less than allof the zones) and through the thin jacket during the purge stage, theliquid temperature rapidly decreases to a minimum operating temperatureT_(L) (e.g., in 2.3 minutes, as shown at t₁₃.5 to t₁₅.8 in FIG. 6) andthe burner is turned on again. Since the BTU output of the burner ishigh relative to the BTU capacity of the liquid in the thin jacket andrelative to the rate of heat transfer from the liquid-to-air heatexchangers of the active zones, the temperature of the liquid increasesrapidly in time period t₁₅.8 to t₁₇ in FIG. 6 (1.2 minutes). Thetemperature of the liquid again reaches the high temperature limit T_(M)in this short time period even though there is a 14,000 BTU per hourthermal demand. The burner is again shut off and cycled during the timeperiod t₁₇ to t₁₉.3 through the abbreviated purge stage before theburner operates in the optimum stage. This cycle of start-up, too-shortor no optimum stage operation and abbreviated purging continues,resulting in low burner efficiency, requirements for frequent cleaningof the combustion chamber and decreased burner life due to thermalstresses. Even if the difference between T_(M) and T_(L) were doubled to30° F. to correspond to the example used in FIG. 7, it would take only2.4 minutes of burner operation for the temperature to reach T_(M).

In applicant's experience, attempts to lengthen the optimum burningstage have resulted in overall inefficiency of the system. For example,some vehicles having many zones for room air heating have used adiverter valve between the thin jacket and a return conduit from theliquid-air heat exchanger in the active zone. The valve is controlled todivert the liquid into a heat exchanger in the main power source, suchas in the main internal combustion engine. Heat is lost in the mainengine heat exchanger even though there may be no need to heat the mainengine. This of course wastes heat, but lowers the temperature of theliquid that is input to the thin jacket, and thus increases the durationof the optimum stage of burner operation. Because this method ofoperating the supplemental burner depends on wasting the excess heatfrom the burner, it is not an acceptable way of increasing the length ofthe optimum stage of burner operation.

The method and apparatus of the present invention overcome theabove-described rapid cycling of the supplemental burner without wastingthe excess heat produced by the supplemental burner. In particular, thesupplemental burner may operate at its rated thermal output, which mayexceed the thermal demand of the active room air heating zones, yet thesupplemental burner operates for an acceptably long, three-stageoperational cycle (illustrated by t₄ to t₁₆ in FIG. 7). This is achievedby providing a substantially increased mass (and volume) of heattransfer liquid in heat transfer relationship with the combustionchamber of the supplemental burner. This increased mass (and volume) ofliquid is heated by the supplemental burner to the maximum operatingtemperature T_(M) to provide a reservoir of thermal energy. When thereis demand for heat in a single zone, or in less than all of the zones ofthe room-air heat transfer system, or when there is only a small demandfor domestic hot water, the thermal energy required to meet that demand(which is selected as 14,000 BTU per hour in the situation depicted inFIG. 7) is initially supplied by heat transferred from the reservoir ofthermal energy. As this initial heat transfer occurs (see t₁₆ to t₃₇ inFIG. 7) from the thermal reservoir to the room air heat exchanger, or tothe domestic water, the supplemental burner remains off. As a result, inone embodiment of the present invention 5100 BTUs are supplied by thethermal reservoir before the temperature of the heat transfer liquid inthe thermal reservoir falls to the lower operational temperature, or lowlimit T_(L). When that low limit T_(L) is reached at the time t₃₇, theburner is turned on. Even if there is no additional demand for thermalenergy for room air heating or for domestic water heating, the thermalload of the large mass and volume of heat transfer liquid, expressed interms of the BTU requirement to heat the large mass and volume of heattransfer liquid back to the maximum operating temperature T_(M), isenough that the supplemental burner must operate at its rated capacityfor an acceptably long operational cycle, including an acceptably longoptimum cycle. That operational cycle is shown in FIG. 7 as the timeperiod t₃₇ to t₄₄.65, based on the use of fourteen gallons of heattransfer liquid in the thermal reservoir and a 40,000 BTU per hour ratedcapacity of the supplemental burner. The supplemental burner thusoperates for about 7.65 minutes (time t₃₇ to time t₄₄.65) just toincrease the temperature of the heat transfer liquid in the thermalreservoir from 150° F. (T_(L)) to 180° F., 180° F. being the maximumoperating temperature T_(M) of the thermal reservoir. With continueddemand for heated room air or for domestic hot water, it would takelonger than 7.65 minutes to satisfy that demand and to increase thetemperature of the liquid in the thermal reservoir to the maximumoperating temperature T_(M). Thus, the supplemental burner would operatefor a longer period of time in the optimum stage, which is the mostefficient stage, and would burn the carbon off the inside of thecombustion chamber. In addition to increasing the duration of theoptimum stage of the burner's three-stage operational cycle, the 5100BTUs of thermal energy that are stored in the thermal reservoir increasethe ability of the system of the present invention to satisfy thermaldemands that exceed the thermal output of the supplemental burner.

An object of the present invention is to increase the operatingefficiency of an auxiliary burner.

Another object of the present invention is to provide a thermalreservoir heated by an auxiliary burner to provide a selected amount ofthermal energy to meet demands for initially heating less than all ofthe zones in a zoned room air heating system without operating theauxiliary burner.

A further object of the present invention is to provide a thermal energystorage reservoir having a selected thermal capacity, where that thermalcapacity is met by operating an auxiliary burner for a time period notless than its minimum desired operational cycle.

A still other object of the present invention is to provide a method ofusing an auxiliary burner to initially supply thermal energy to athermal reservoir to increase its temperature to a maximum operatingtemperature, and to use that reservoir, and not the burner, to supplythermal energy to a room air and domestic water heating system until thetemperature of the reservoir drops to a minimum operating temperature,at which time the auxiliary burner is operated for at least the durationof a minimum desired operational cycle to reheat the thermal reservoirto its maximum operating temperature.

A still further object of the present invention is to provide a thermalenergy reservoir having a given thermal storage capacity, the reservoirbeing heated by an auxiliary burner having a minimum desired operationalcycle that is of a selected minimum length at its rated thermal output,where thermal energy equal to that thermal storage capacity is providedto the reservoir by the auxiliary burner operating for no less than theduration of such minimum desired operational cycle.

With these and other objects in mind, the present invention is used withan auxiliary burner that provides supplemental thermal energy to avehicle having a room or zone to be heated and domestic water to beheated. The auxiliary burner is designed to operate efficiently at arated thermal output when it is operated for no less than the durationof a minimum desired operational cycle. A reservoir is provided forcontaining heat transfer liquid having a selected thermal energy storagecapacity, measured by the amount of thermal energy required to heat theliquid from a lower or minimum operating temperature to a maximumoperating temperature. The heat transfer liquid is in heat transferrelationship with the auxiliary burner and the thermal energy storagecapacity is selected so that the auxiliary burner must operate at itsrated thermal output for at least the duration of the minimum desiredoperational cycle to provide the selected thermal energy storagecapacity to the reservoir.

In a method of the present invention, the reservoir containing the heattransfer liquid having the selected thermal energy storage capacity isinitially heated by the auxiliary burner operating at the rated thermaloutput for at least the duration of the minimum desired operationalcycle. With the heat transfer liquid in the thermal reservoir at themaximum operating temperature and with the burner off, heat istransferred from the thermal reservoir to meet the initial thermaldemands for heated room air and domestic hot water. These demandsdecrease the temperature of the heat transfer liquid to the minimumoperating temperature, whereupon the auxiliary burner is operated for atleast the duration of the minimum desired operational cycle to raise thetemperature of the heat transfer liquid to the maximum operatingtemperature. If during the operation of the auxiliary burner there aredemands for thermal energy for heating room air or domestic water, theauxiliary burner is operated for a time period longer than the durationof the minimum desired operational cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention will beapparent from an examination of the following detailed descriptions,which include the attached drawings in which:

FIG. 1 is a perspective view of a vehicle provided with an auxiliaryburner for providing supplemental thermal energy for room air, domesticwater and main engine heating;

FIG. 2 is a perspective view of the auxiliary burner provided with athermal reservoir of the present invention, wherein the thermal energystorage capacity of the reservoir is related to the duration of theminimum desired operational cycle of the auxiliary burner so that theauxiliary burner operates for at least that minimum duration regardlessof the actual thermal demands to be met by the thermal reservoir;

FIG. 3 is an end elevational view and schematic diagram of the conduitsthat are connected to the thermal reservoir of the present invention;

FIG. 4 is a side elevational view of the thermal reservoir shown in FIG.3, illustrating a domestic water coil in heat transfer relationship withthe thermal reservoir;

FIG. 5 is a cross-sectional view taken along lines 5--5 in FIG. 4showing a coil in the thermal reservoir transferring thermal energy fromor to heat transfer liquid from a main engine of the vehicle;

FIG. 6 is a graph showing temperature vs. time illustrating typicaloperational cycles of a prior art auxiliary burner, which cycles are ofundesired length; and

FIG. 7 is a graph of temperature vs. time illustrating typicaloperational cycles of a burner of the present invention, wherein thosecycles exceed the duration of a minimum desired operational cycle ofsuch burner.

DETAILED DESCRIPTION Vehicle 20

Referring to FIG. 1, there is shown a vehicle 20 that is designed to bepropelled by a main engine 21. As described above, the vehicle 20 mayalso be in the form of a boat, in which even the main engine 21 propelsthe boat on the water. The vehicle 20 may also be a trailer that istowed by another self-propelled vehicle.

The main engine 21 may be an internal combustion engine or other type ofengine having a liquid coolant system 22 for maintaining the main engine21 at a desired operating temperature (T_(EO)). When the vehicle 20 isbeing propelled by the main engine 21, thermal energy is supplied via aconduit 23 that carries heated engine coolant 24 (FIG. 3) to anauxiliary thermal energy system 25. Cooled coolant 24 (FIG. 3) isreturned to the main engine 21 via a return conduit 26. When the mainengine 21 is not operating, the main engine 21 is maintained at adesired idle temperature (T_(EI)) by supplying heated engine coolant 24from the auxiliary system 25 to the main engine 21 via the returnconduit 26.

In the various forms of the vehicles, separate spaces or rooms 27(illustrated by dashed lines in FIG. 1) are provided for various livingor working activities. In each room, at least one liquid-to-air heatexchanger 28 is provided for heating the room air to a desiredtemperature (T_(A)). These heat exchangers 28 may be of the standardtype sold under the trademark Aurora as motor home heaters. Heattransfer liquid 29 (FIG. 2) is supplied to the heat exchangers 28 fromthe auxiliary system 25 by separate supply conduits 30 and is returnedto the auxiliary system by separate return conduits 31.

As shown in FIG. 1, one of the rooms 27 may be a bathroom 32 that isprovided with a shower head 33. Another room 27 may be a kitchen 34provided with a sink 35 and faucet 36. The shower head 33 and the faucet36 are connected to a standard domestic water tank 37. As shown, a pump38 provides pressure to supply domestic water 39 from the tank 37. Thedomestic water 39 is heated by the auxiliary system 25 and is suppliedvia hot water conduits 40 to the respective shower head 33 and thefaucet 36.

Referring to FIG. 2, the supply and return conduits 30 and 31respectively, that are connected to the room air heat exchangers 28 areshown connected to zone pumps 41, 42, and 43. A manifold conduit 44(FIGS. 2 and 3) supplies heated heat transfer fluid 29 (FIG. 4) to thezone pumps 41, 42, and 43 from the auxiliary system 25. In a typicalvehicle 20, a zone to be heated is defined by a room 27, and may have athermal load of from 3000 to 8000 BTUs per hour. This load representsthe thermal energy necessary to maintain the room air temperature in theroom 27 at 75° F., for example, with an outside ambient temperature offrom 0° to 50° F. In the aggregate, the thermal load of all of the rooms27 of the vehicle 20 would typically be about 20,000 BTUs per hour.

Referring to FIG. 1, a cold water conduit 45 from the domestic watertank 37 is shown connected to the auxiliary system 25 to provide heateddomestic water 39 (FIG. 1) in the hot water conduit 40 that is connectedto the shower head 33, a lavatory 47 (FIG. 1) and the kitchen faucet 36.A mixing valve 45A (FIG. 2) is shown for blending hot domestic water 39from the conduit 40 with cold domestic water 39 from the cold waterconduit 45 to obtain a desired temperature of the hot domestic water 39exiting the system 25. The typical demand for hot domestic water 39 isabout 1.5 gpm for the shower head 33, the kitchen faucet 36 and thelavatory 47 at a temperature of 105° F., for example. If the colddomestic water 39 is stored in the tank at 55° F., for example, then thethermal load of that domestic water 39 would be about 40,000 BTU perhour.

BURNER 48

Referring to FIGS. 2-5, to supply the thermal load of such room air heatexchangers 28 and the domestic hot water 39, a vehicle 20 such as a"recreational vehicle," for example, is provided with the auxiliarysystem 25 having a peak thermal output of about 40,000 BTU per hour. Theauxiliary system 25 may include a propane, gasoline or diesel-firedburner 48. In a preferred embodiment, the burner 48 is a Model DBW 2010burner manufactured by Webasto AG having a thermal output of 11.6 kw(40,000 BTU/hr.). Such a burner 48 is normally shipped with a combustionchamber 49 (FIG. 5) in the form of a closed horizontal cylinder 50having an air/fuel inlet 51 at one end 52 and an exhaust pipe 53 (FIG.3) at the other end 54. The combustion chamber 49 of the burner 48 isabout twelve inches long and has an o.d. of about six inches. A coolantjacket (not shown) of such burner 48 is a cylinder having a diametertypically one inch greater in diameter than that of the o.d. of thecombustion chamber 49 (or seven inches) and a length of about twelve andone-half inches. Such a coolant jacket holds about 0.26 gallons of heattransfer liquid 29. As an example, the thermal energy storage capacityof that amount of liquid, measured as the amount of thermal energyrequired to heat 0.26 gallons of the liquid 29 ethylene glycol from 155°F. to 170° F. (a typical ΔT of T_(M) -T_(L) in the prior art) is 36BTUs. Such a burner 48 with the described coolant jacket (not shown) istypically operated with a liquid flow rate of 4.5 gpm and can provide an18° F. gain in temperature (ΔT) of the liquid 29 in one pass through thecoolant jacket (not shown). It is to be understood then that such"thermal energy storage capacity" of that amount of liquid representsthe amount of thermal energy in BTUs stored by that particular liquidwith respect to such ΔT. That amount of thermal energy is transferredfrom the particular liquid as the temperature of the liquid drops from170° F. to 155° F.

In the use of burners 48 of the type illustrated by the Webasto ModelDBW 2010, it has been the practice to recommend preferred operatingparameters. Among these parameters is a preferred minimum desiredoperational cycle having a preferred minimum duration. This cycleincludes the start-up stage described above, which includes starting thepump 41, 42, or 43 for the particular room air heating zone or room 27and then starting combustion. As indicated above, the start-up stagealso includes an initial portion of the period in which combustionoccurs. In this initial portion, which may take three minutes, theburner 48 is not running at the intended efficiency because thecombustion chamber 49 (FIG. 5) is heating up, for example. In FIGS. 6and 7, for purposes of illustration, the thermal output of the burner 48is considered as being the same during the initial stage and during theoptimum stage. It is to be understood that such thermal output will besomewhat less during the initial stage.

Once the combustion chamber 49 (FIG. 5) has reached its operatingtemperature (T_(BO)), the combustion becomes complete according todesign specifications of the burner 48. At this time, the carbon thatwas deposited on an inner wall 55 (FIG. 5) of the combustion chamber 49starts to burn off. As the complete combustion continues, more of thecarbon burns off, so that after a period of two minutes of optimumoperation, the combustion chamber 49 is relatively clean and heat ismore readily transferred across the wall 55 of the combustion chamber 49to the heat transfer liquid 29. At this time, after an optimum stage ofno less than about two minutes, the burner 48 may be turned off and thepurge stage is completed in about two and one-half minutes to cool downthe burner 48. Thus, the preferred minimum duration of the minimumdesired operational cycle is about seven and one-half minutes inapplicant's experience using the Model DBW 2010.

The thermal output of the burner 48, measured by the heat transferred tothe heat transfer liquid 29 during the minimum desired operationalcycle, is determinable and is referred to as the "rated thermal outputfor minimum desired operational cycle, " or "RTO." In the example of theModel DBW 2010, the RTO is about 3333 BTUs. This is obtained bymultiplying the 40,000 BTU per hour rate of thermal output by theduration of the start-up and minimum desired optimum stage (threeminutes, plus two minutes, respectively).

Auxiliary Thermal Energy System 25 of the Present Invention

To avoid the disadvantages of using the burner 48 with small volumeliquid jackets (not shown) which are described above as having a liquidcapacity such as 0.26 gallons, according to the principles of thepresent invention a standard burner 48, is modified by removing thesmall volume jacket. In its place, a substantially larger thermalreservoir 56 (FIGS. 2 and 4) is mounted around the combustion chamber 49(FIG. 5) and contains a selected mass and volume of the heat transferliquid 29, such as ethylene glycol. The rated thermal output of theburner 48 during the minimum desired operational cycle (or RTO) isknown, as described above.

The desired operational temperature range (Δ T_(R), in °F.) of the heattransfer liquid 29 in the thermal reservoir 56 is selected based on thedesired T_(M) and T_(L) for heat transfer purposes, where Δ T_(R) =T_(M)-T_(L). This is shown as 30° F. in FIG. 7. Knowing the specific heat(S.H.) of the liquid in BTUs per pound of liquid per degree F, theminimum weight of the liquid 29 is determined by solving the followingequation for W_(L), the weight of the liquid 29:

    RTO=(ΔT.sub.R)(S.H.)(W.sub.L)                        (1)

W_(L) can be expressed in other terms as that weight of heat transferliquid 29 that is raised through a desired Δ T_(R) by the burner 48operating for the duration of the minimum desired operational cycle atthe rated thermal output of the burner 48.

As an example, in a preferred embodiment of the present invention usinga Model DBW 2010 burner, the liquid capacity of the reservoir 56 isfifteen gallons and is filled with fourteen gallons of the liquid 29 toallow one gallon for expansion. The reservoir 56 is provided as acylinder having an i.d. of sixteen inches and a length of eighteeninches, and surrounds the combustion chamber 49 of the burner 48. Thereservoir 56 is fabricated from mild steel having a weight of aboutfifty-four pounds and a specific heat of one BTU/pound/°F. To minimizethe weight W_(L) of the heat transfer liquid 29 that is required, thethermal energy storage capacity of the reservoir 56 can be added toequation (1) as follows, where "S.H._(M) " is the specific heat of thematerial from which the reservoir 56 is made, Δ T_(M) is the same as ΔT_(R) used in equation (1), and W_(R) is the weight of the emptyreservoir 56:

    RTO=(ΔT.sub.R)(SH.sub.L)(W.sub.L)+(ΔT.sub.M)(SH.sub.M)(W.sub.R)(2)

It is to be understood then, that the thermal energy storage capacity ofthe thermal reservoir 56 may be generally expressed in terms of Equation(1) and may be more particularly expressed in terms of Equation (2). Inboth cases, the amount of thermal energy stored in the reservoir 56 isequal to RTO.

The thermal reservoir 56 of the present invention is connected to theauxiliary system 25 in three ways. First, the manifold conduit 44 isconnected to an outlet port 57 (FIG. 3) of the thermal reservoir 56located toward the top of the thermal reservoir 56. The manifold conduit44 supplies the heated liquid 29 to each of the three zone pumps 41, 42and 43. A selected one or more of the pumps 41, 42 and 43 is operated tosupply the heated liquid 29 to the heat exchanger 28 in the zone or room27, such as the kitchen 34 to which the supply conduit 30 is connected.The liquid 29 exits the heat exchanger 28 and returns via the returnconduit 31 to an inlet 58 (FIG. 4) at the bottom of the thermalreservoir 56.

Second, the domestic water 39 is supplied from the domestic water tank37 by the pump 38. The cold water conduit or pipe 45 is connected to thepump 38 and supplies cold domestic water 39 (at 55° F.) to an inlet 59(FIG. 4) of a coil 60 located at the bottom 61 of the thermal reservoir56. The coil 60 is secured, such by brazing, in a serpentine path (FIGS.3 and 4) or in a circular path (FIG. 2) to the outer side 62 of thethermal reservoir 56 so that the domestic water 39 in the coil 60 is inheat transfer relationship with the liquid 29 in the thermal reservoir56. The pump 38 causes the domestic water 39 to flow through the coil 60to the hot water line or conduit 40 that supplies the domestic hot water39 to the kitchen faucet 36, the shower head 33 and the lavatory 47.

Third, the respective vehicle engine supply and return conduits 23 and26 are connected to a respective coil 63 (FIG. 5) and an engine coolantpump 64 (FIG. 2). The pump 64 causes the engine coolant 24 to flowthrough the heat exchange coil 63 that extends through the liquid 29 inthe thermal reservoir 56 and to the return conduit 26 to the main engine21. If the main engine 21 is to be heated, the liquid 29 is in a desiredrange of from 150° F. to 180° F. If the liquid 29 in the thermalreservoir 56 is to be heated during operation of the main engine 21, thecoolant 24 is at a higher temperature than that of the liquid 29 in thethermal reservoir 56, such as 190° F.

The auxiliary system 25 of the present invention is also provided withan electric heater 65 (FIGS. 4 and 5) to maintain the liquid 29 in thethermal reservoir 56 in a ready condition at the upper or maximumoperating temperature (T_(M)) of 180° F. Since heat transfer occursacross insulation 66 around the thermal reservoir 56 and across theconduits 40, for example, thermal energy must be transferred to theliquid 29. The electric heater 65 has a rated capacity of 1500 Watts at115 volts ac, which is sufficient to supply enough thermal energy to theliquid 29 to offset that lost by heat transfer across the insulation 66.A standard ac generator or power supply (not shown) is provided forsupplying power to the heater 65.

Referring to FIG. 3, the reservoir 56 is shown provided with first andsecond thermostats 67 and 68. The thermostats 67 and 68 extend into theheat transfer liquid 29 in the reservoir 56 for response to thetemperature T_(R) of the liquid 29. Each thermostat 67 and 68 may be aconventional Robertshaw thermostat which responds to the maximumtemperature T_(M) of the liquid 29 by opening a circuit 69 or 70respectively. The circuit 69 is connected to the burner 48. When thecircuit 69 is open, the burner 48 shuts off and the purge stage starts.In the example described above, the T_(M) at which the thermostats 67and 68 open the respective circuits 69 and 70 is 180° F. The circuit 70is connected to the electric heater 65. When the circuit 70 is open, thepower to the heater 65 is shut off.

Each of the thermostats 67 and 68 can also be set to close therespective circuits 69 and 70 (FIG. 3) in response to the liquid 29having the lower limit temperature T_(L). In the example describedabove, where T_(L) is 150° F., the thermostat 68 for the electric heater65 closes the circuit 70 in response to a temperature of 155° F. of theliquid 29, whereas the thermostat 67 closes the circuit 69 in responseto a temperature of 150° F., which is T_(L). In this manner, theelectric heater 65 operates first in an attempt to provide thermalenergy to the liquid 29 to offset heat lost across the insulation 66,for example. When the temperature of the liquid 29 drops to T_(L), thenthe burner 48 is turned on and the temperature of the liquid 29 isincreased to T_(M) during the minimum operational cycle. Since thethermal output of the electric heater 65 is about 5118 BTU/hour, theelectric heater 65 can be left on during the minimum operational cycle,with the 7.65 minute burner operational cycle being reduced to over 6minutes, which exceeds the minimum desired operational cycle.

Thermal Reservoir 56

The method and apparatus of the present invention overcome theabove-described rapid cycling of the supplemental burner 48 withoutwasting the excess heat produced by the supplemental burner 48. Inparticular, the supplemental burner 48 may operate at its rated thermaloutput, which exceeds the thermal demand of the active room air heatingzones 27, yet the burner 48 operates during the minimum desiredoperational cycle. This is achieved by providing the substantiallyincreased weight W_(L) (or volume) of the heat transfer liquid 29 inheat transfer relationship with the combustion chamber 49 of the burner48. The weight W_(L) is determined by reference to equations (1) or (2)above. This increased weight W_(L) of liquid 29 is heated by thesupplemental burner 48 to the maximum operating temperature (T_(M)) toprovide a reservoir of thermal energy (see time t₁₆ in FIG. 7). Thethermal energy storage capacity of such reservoir of thermal energy isthe RTO of the burner 48. When there is demand for heat in a single zone27, or less than all of the zones 27 of the vehicle 20, or when there isonly a small demand for domestic hot water 39, the thermal energyrequired to meet that demand is initially supplied by heat transferredfrom the reservoir 56 of thermal energy. As this initial heat transferoccurs from the thermal reservoir 56 to the room air heat exchanger 28,or to the domestic water 39, the supplemental burner 48 remains off. Asa result, the entire thermal energy storage capacity of the liquid 29 inthe reservoir 56 up to 5100 BTUs in the example given) can be suppliedby the thermal reservoir 56 before the temperature of the heat transferliquid 29 in the thermal reservoir falls to the lower limit or loweroperating temperature (T_(L)). Further, when the weight W_(R) of thematerial of the reservoir 56 is considered, as in equation (2),additional thermal energy is provided as that material cools to T_(L).Thus, as the liquid 29 and the material of the reservoir 56 decrease intemperature from T_(M) to T_(L), the increased thermal energy storagecapacity of the reservoir 56 results in providing thermal energy equalto the RTO of the burner 48.

When that limit T_(L) is reached, the burner 48 is turned on. Even ifthere is no additional demand for thermal energy for room air heating orfor domestic water heating, the thermal load of the large volume of heattransfer liquid 29 and the material of the reservoir 56, if it isconsidered, required to heat it or them back to the maximum operatingtemperature (T_(M)) is enough that the supplemental burner 48 mustoperate at its rated capacity for at least the duration of the minimumdesired operational cycle, including an acceptably long optimum cycle(see times t₃₇ to t₄₄.65 in FIG. 7). As an example, with fourteengallons of ethylene glycol as the heat transfer liquid 29 and a 40,000BTU per hour rated capacity of the supplemental burner 48, thesupplemental burner 48 operates for 7.65 minutes to increase the thermalreservoir temperature (T_(R)) from the 150° F. minimum operatingtemperature T_(L) to the maximum operating temperature T_(M) of 180° F.With continued demand for heated room air or for domestic hot water 39,it would take longer to satisfy that demand and to increase thetemperature T_(R) of the thermal reservoir 56 to the maximum operatingtemperature T_(M). Thus, the supplemental burner 48 would burn longer inthe optimum stage, which is the most efficient stage, and would burn thecarbon off the inside of the combustion chamber 49. In addition toincreasing the duration of the burner's optimum stage, the 5100 BTUs ofthermal energy that are stored in the thermal reservoir 56 increase theability of the system 25 of the present invention to satisfy thermaldemands that exceed the thermal output of the burner 48.

METHODS OF THE PRESENT INVENTION

One aspect of the present invention relates to the selection of theburner 48 having a minimum desired duration operational cycle. Thethermal output RTO of the burner 48 during that minimum operationalcycle is determined. The liquid for the heat transfer liquid 29 isselected, which determines the specific heat S.H. in equation (1). The ΔT_(R) is then selected based on the required maximum operatingtemperature for heat transfer to the liquid 29 from the combustionchamber. Equation (1) is then solved for W_(L) which determines theweight of the liquid 29 that is to be contained in the thermal reservoir56. The thermal output RTO of the burner 48 is thus used to select thethermal energy storage capacity of the volume of heat transfer liquid 29that is provided in the thermal energy storage reservoir 56. Thatthermal capacity is equal to the thermal output RTO of the burner 48according to equation (1).

In another aspect of the method of the present invention, the reservoir56 containing the heat transfer liquid 29 having the selected thermalenergy storage capacity is initially heated by the auxiliary burner 48operating at the rated thermal output for at least the duration of theminimum desired operational cycle. That duration of heating provides thethermal output RTO and results in heating the heat transfer liquid 29 inthe thermal reservoir 56 to the maximum operating temperature T_(M) withno thermal demands on the reservoir 56. With the heat transfer liquid 29in the thermal reservoir 56 at the maximum operating temperature T_(M),heat is transferred from the thermal reservoir 56 to meet thermaldemands, e.g., for heated room air and domestic hot water 39. Since thethermostats 67 and 68 open the respective circuits 69 and 70 attemperatures above T_(L), no thermal energy is supplied by the burner 48to the heat transfer liquid 29 at this time. Thus, these demandsdecrease the temperature of the heat transfer liquid 29 to the minimumoperating temperature T_(L). The auxiliary burner 48 is then renderedoperative when the thermostat 67 senses the liquid at T_(L) and closesthe circuit 69. The auxiliary burner 48 is operated for at least theduration of the minimum desired operational cycle to raise thetemperature of the heat transfer liquid 29 to the maximum operatingtemperature T_(M). If during the operation of the auxiliary burner 48there are demands for thermal energy for heating room air or domesticwater 39, the auxiliary burner 48 is operated for a time period longerthan the duration of the minimum desired operational cycle, as shown inFIG. 7 by the time period between t₄ and t₆. When the temperature of theliquid 29 reaches T_(M), the thermostat 67 causes the circuit 69 to openand the burner 48 is rendered inoperative, which is to say the optimumstage stops and the purge stage starts. The burner 48 remains off untilthe temperature of the liquid 29 drops to T_(L) (see time t₃₇ in FIG.7).

The demand for thermal energy can also result from a need to maintainthe main engine 21 at the desired idle temperature T_(EI). In thisevent, a switch 71 is closed to open a valve 72 and energize the enginecoolant pump 64. The pump 64 circulates the engine coolant 24 from thereturn conduit 26 to the heat transfer coil 63 for the coolant 24. Thecoil 63 receives thermal energy from the heat transfer liquid 29 andheats the coolant 24. The coolant 24 is pumped through the supplyconduit 23 to the main engine 21 where it circulates within and heatsthe main engine to the temperature T_(EI).

When the main engine 21 is operating, the flow of coolant 24 can bereversed to provide heated coolant 24 to the coil 63 for heating theliquid 29. In the event that the thermal demand exceeds the thermalenergy supplied by the heated coolant 24 in the coil 63, the burner 48turns on when the thermostat 67 senses T_(L) of 150° F.

While the preferred embodiments have been described in order toillustrate the fundamental relationships of the present invention, itshould be understood that numerous variations and modifications may bemade to these embodiments without departing from the teachings andconcepts of the present invention. Accordingly, it should be clearlyunderstood that the form of the present invention described above andshown in the accompanying drawings is illustrative only and is notintended to limit the scope of the invention to less than that describedin the following claims.

What is claimed is:
 1. A method of selecting heat transfer liquid foruse in a burner of an auxiliary heating system, comprising the stepsof:determining the thermal output of said burner during a minimumoperational cycle of said burner; selecting heat transfer liquid havinga thermal energy storage capacity equal to said thermal output of saidburner; and providing said heat transfer liquid in heat transferrelationship with said burner.
 2. A method of selecting the weight ofheat transfer liquid to be contained in a thermal reservoir in heatexchange relationship with a burner that provides thermal energy,comprising:selecting the material to be used as said heat transferliquid; determining the specific heat (S.H._(L)) of said material ofsaid liquid; selecting a temperature differential (Δ T_(R)) for heattransfer from said burner to said liquid; determining the thermal output(RTO) of said burner during the duration of a minimum desiredoperational cycle of said burner; selecting the material to be used assaid thermal reservoir; determining the specific heat (S.H._(R)) of saidmaterial for said reservoir; selecting the weight W_(R) of said thermalreservoir; and determining the weight (W_(L)) of said liquid by solvingthe following equation for W_(L) :

    RTO=(ΔT.sub.R)(S.H..sub.L)(W.sub.L)+(ΔT.sub.R)(S.H..sub.R)(W.sub.R).


3. The method of claim 1, in which:said step of selecting said heattransfer liquid includes selecting a given weight of said heat transferliquid, selecting said heat transfer liquid having a particular specificheat, and selecting an operational temperature range over which saidheat transfer liquid is heated during the minimum operational cycle. 4.The method of claim 1, comprising the following further steps ofcontrolling the operation of said burner:with said heat transfer liquidinitially at a maximum operational temperature and with said burner notoperating, transferring heat from said heat transfer liquid to decreasethe temperature of said heat transfer liquid to a minimum operationaltemperature; and operating said burner for a duration of not less thanthe duration of the minimum operational cycle to increase thetemperature of the heat transfer liquid to the maximum operationaltemperature.
 5. The method according to claim 1, further comprising thefollowing step of controlling the operation of said burner:operatingsaid burner in cycles of no less duration than the duration of theminimum operational cycle.
 6. A method of selecting the weight of heattransfer liquid to be contained in a thermal reservoir in heat exchangerelationship with a burner that provides thermal energy to said thermalreservoir, comprising:selecting the material to be used as said heattransfer liquid; determining the specific heat (S.H._(L)) of saidmaterial of said liquid; selecting a temperature differential (Δ T_(R))for heat transfer from said burner to said liquid; determining thethermal output (RTO) of said burner during the duration of a minimumdesired operational cycle of said burner; and selecting the weight ofsaid liquid by solving the following equation: ##EQU1##